1. Field of the Invention
The present invention relates to an optical communication module applied to the wavelength division multiplexing optical communication system, That is, pertaining to a device for locking an oscillating wavelength of a laser luminous source to a set point wherein an optical system to operate stably against the change of the ambient temperature and its control system are provided. This optical system can operate separately as a wavelength locking module, but it may be incorporated into an optical communication module provided with a laser luminous source.
2. Description of the Related Art
Optical fiber communication enables us to construct a highly credible communication system, as it is excellent in long distance, high-speed and large volume transmission as well as electromagnetic noise resistance. In this system, one wavelength of light is carried on each optical fiber, but further increase of transmission volume is solicited by the concerned according to the advent of the recent large volume transmission environment. Thus, in order to realize large volume transmission by carrying a number of lights having different wavelengths on each optical fiber so as to increase the number of communication channels, nowadays, the wavelength division multiplexing optical communication system is put into practice. For the wavelength of light to be transmitted through an optical fiber, the wavelength bands of less transmission loss of the fiber are used, the 1.3 and 1.5 micron areas of which wavelength bands are called windows of transmission. The wavelength width of the respective windows is limited to some extent, so that the narrower the wavelength interval between adjacent channels becomes, the more the number of transmission channels increases. At present, the frequency intervals are represented with 200 GHz and 100 GHz, which intervals tend to be narrower, e.g., 50 GHz, 25 GHz and further, which correspond to approximately 1.6 nm, 0.8 nm, 0.4 nm, 0.2 nm, 0.1 nm if represented by the wavelength intervals. When the wavelength intervals become narrow as described above, it requires that the wavelength of the laser luminous source be kept constant with precision. That is because the crosstalk with adjacent channels occurs on the part of the receiver when the wavelength of the laser luminous source unstably reaches that of adjacent channels, with the result that the credibility of the communication is not guaranteed. Those wavelength (or frequency) channels are called xe2x80x9cITU-T (International Telecommunication Union-Telecommunication Standardization Sector) gridxe2x80x9d, which is widely recognized as an International Telecommunication Union Standard.
In view of the foregoing, various methods for controlling the wavelength of the laser luminous source of the wavelength multiplexing optical communication have been proposed to date. As a wavelength selection device, for examples, a dielectric filter or a Fabry-Perot etalon is introduced so as to perform a feedback operation on the operating temperature of the semiconductor laser and to fix a wavelength. Especially, the Fabry-Perot etalon has the characteristics that transmission peaks repeatedly appear according to the degree of multiplex interferences, so that the accordance of the period of the transmission curve with the ITU grid enables the plurality of wavelength channels to be locked just with the sole selective filter element. For instance, according to the disclosure of Japanese Patent Application Laid-open No. HEI 10-79723, it is proposed that a wavelength is locked by separating the light transmitted through the etalon into two divisions, which divisions are received by a light detector so as to utilize the difference in wavelength between those divisions as a wavelength error signal.
However, the above Fabry-Perot etalon has the characteristic dependency on the ambient temperature. That is, it has the characteristics that the optical path length of the etalon changes and the wavelength of the transmitted light of the etalon varies owing to the change of its refractive index and coefficient of linear expansion when the ambient temperature of the etalon changes. In other words, it has been found that the change of the ambient temperature causes a gap between the wavelength point as desired through the utilization of the etalon and the actual wavelength to be locked. This gap arises from when carrying out a so-called tracking test or the ambient temperature of the package incorporating a wavelength locker therein changing from 0 to 70 centigrade. The present invention is to abate the temperature dependency of the etalon.
The means to solve the prior issue as described above according to the present invention is as follows. The etalon which corresponds to a wavelength error detecting device is installed on a substrate which is placed higher than the portion where the optical system of semiconductor laser convergence including a holder is installed and which contacts one end of a electronic cooler. Alternatively, the etalon is covered with a metallic material of high heat conductivity so that the temperature distribution within the etalon is of the minimum or minor range.